A peculiarity of form, character, or tendency to disease has been known to disappear in one generation and re-appear in the next; this variety of heredity is termed _atavism_. The evidence showing the inheritance of acquired characters, _i.e._ those which arise in consequence of the effect of external forces on the organism is not conclusive. Weismann believes that only those forces that influence the germ-plasm are inherited. It must be admitted that the instances of inheritance of acquired characters can be better explained otherwise. Thus the long neck of the giraffe was formerly explained on the supposition that the neck became gradually lengthened owing to the efforts made generation after generation in reaching food; but is better explained by Weismann on the supposition that those giraffes which, during times of famine were able to reach higher and obtain food from the twigs of trees would survive and pass on their characteristics to their young, while shorter necked giraffes would be exterminated. The inheritance of proclivity to or immunity from attacks of infectious diseases is a problem of great difficulty; but there is no substantial reason for thinking that the efforts being made to diminish the prevalence of these diseases (including consumption) are likely to produce a weaker race or one more likely to suffer with excessive severity from these diseases should they be introduced after a long absence. (See also page 309). =Temperament= indicates a peculiarity in constitution, causing a liability to particular diseases, or to a special character in any disease to which a person becomes subject. Four temperaments are usually recognized—the sanguine, phlegmatic, bilious, and nervous, but unmixed specimens of these temperaments are rarely seen. By =idiosyncrasy= is understood a peculiarity limited to a comparatively small number of individuals. Four varieties of idiosyncrasy may be described. The first consists in an extreme susceptibility to the action of certain things, or an extreme lack of susceptibility. Thus most people at some time or other inhale the pollen of grasses, but only in a few cases does it produce that troublesome and distressing complaint—hay asthma. In certain persons a very minute dose of iodide of potassium produces distressing symptoms; in most cases these symptoms arise if the drug is taken for a prolonged period; but in a few cases it may be taken for an indefinite period without troublesome result. The case of a physician at Bath is very curious. The smell of hyacinths in bloom always made him faint away; so constant was this result, that before entering a room during the hyacinth season, he always asked the servant if there were any hyacinths in it.